Resists are herein defined as materials which change their solubility to a developer solution upon exposure to light or electron beams. Thus a differential solubility rate between exposed and unexposed portions of a resist film results in the formation of a surface relief pattern after development. Such resists have long been employed in the photolithography art and, more recently, sensitive resists have been used in the preparation of microminiature circuit patterns or other very small information-containing patterns such as holographic patterns.
Photoresists or electron beam-sensitive resists which become more soluble in the exposed regions are known as positive resists and those that become less soluble on exposure are known as negative resists. However, this change in solubility is only a relative change and during solvent development even the less soluble, unexposed portions of the resist dissolve in the developer solvent to some extent. The degree of dissolution of unexposed resist is generally directly proportional to the development time.
The most widely used positive photoresists are of the naphthoquinone (1,2-) diazide sulfonic acid ester type. These photoresists are described for example in U.S. Pat. Nos. 3,046,110, 3,046,111, 3,046,112, 3,046,115, 3,046,118, 3,148,983, 3,184,310 and 3,402,044. Commercially available compositions include, in addition to the naphthoquinone diazide compounds, an alkali soluble novolac resin and other ingredients such as catalysts, sensitizers, etc., in a solvent. Positive photoresists are available from the Shipley Company, particularly AZ 1350 photoresist. In particular, AZ 1350 photoresist is a formulation containing the compound 2,4-bis(6-diazo-5,6-dihydro-5-oxo-1-naphthalenesulfonyloxy)benzophenone ##SPC1##
And a novalac resin. These resists are developed by a basic medium. The above compound admixed in a proportion of about 1 to 50% by weight with an alkali soluble resin produces a formulation found to be particularly sensitive to electron beams, as is disclosed in copending application of Ross and Barton, Ser. No. 332,025, filed Feb. 12, 1973 now U.S. Pat. No. 3,852,771, herein incorporated by reference.
When very thin positive resist films are employed, on the order of 0.5 micron thick films or less, the problem of dissolution of unexposed resist in the developer solvent becomes of increasing importance, particularly when development through to the substrate in the exposed area is desirable. Thus, very careful monitoring of the development process is required to preserve the maximum surface relief of the developed film. The present process provides a method of materially reducing the dissolution rate of unexposed resist during development and increasing the effective sensitivity of the resist.